Replace 'std::mutex' inside 'SyncSwitch' to 'fml::SharedMutex'

[ColdPaleLight]

fix flutter/flutter#102144

SyncSwitch will be used for both frame rasterization on the Raster thread and texture upload on the IO thread.

But SyncSwitch uses std::mutex internally, which means only one thread can access it at a time. This poses a potential risk to the performance of Raster thread.

This PR proposes to change the std::mutex to a fml::SharedMutex.

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[dnfield]

LGTM if LGT @gaaclarke

[gaaclarke]

This is only a win if (time spent waiting on other thread's access to mutex > overhead of SharedMutex).

Given that the IO thread can occupy the mutex for a significant time I think this suggestion is reasonable. It would be nice if we had some sort of data to back up this decision.

Also, when the SyncSwitch was introduced there wasn't so many regions using it, so knowing what we know now it makes more sense. LGTM.